This invention relates to circuit elements usable in optical transmission systems and more particularly to optical wavelength-division multiplexing and demultiplexing devices which are designed to multiplex a plurality of light beams differing from each other in wavelength and demultiplex a light beam including different-wavelength components into beams of respective wavelengths.
As optical fibers, semiconductor lasers and the like are improved in quality and performance, optical-fiber transmission systems are now arousing great interest as promising transmission means in the future and being developed rapidly for practical application. As circuit elements particularly important in forming practical optical-fiber communication systems, optical multiplexing or demultiplexing devices have been required which are designed to couple a plurality of light beams with a single optical fiber or to divide a light beam into a plurality of light beams. Optical multiplexing may be effected in different ways but, among others, the wavelength-division multiplexing technique is suitable for common optical-fiber communications, which employs a plurality of light beams differing from each other in wavelength. For optical wavelength-division multiplexing, it has previously been proposed to direct a plurality of light beams all into a single transmission line by means of a reflecting mirror having a wavelength selectivity or to separate a light beam transmitted through a single transmission channel into beams of respective wavelengths.
Such technique is disclosed in a paper entitled "Optical Waveguide Technology for Modern Urban Communications", by R. L. Gallawa, pp. 131-142, IEEE Transactions on Communications, Vol. COM-23, No. 1, January 1975. The technique, however, is disadvantageous in that it is limited in density of multiplexing or number of transmission channels available on account of the fact that the interchannel wavelength spacing cannot be narrowed as desired since any reflecting mirror whose reflecting power is highly wavelength-dependent is hardly obtainable and that the arrangement of the reflecting mirror in the path of light beam involves an increase in insertion loss. Another disadvantage of this technique is that the structure including the reflecting mirror is rather complicated, lacking stability for any extended period of service.
In an alternative technique previously proposed for optical multiplexing or demultiplexing, use is made of an independent light dispersing element such as a prism or a diffraction grating, as is disclosed, for example, in a paper entitled "Optical Demultiplexing Device Using Prism", by S. Hata, p. 289, Proceedings of the 1976 National Convention of the Institute of Electronics and Communication Engineers of Japan. By this techniques, however, any compact and stable optical wavelength-division multiplexing and demultiplexing device can hardly be realized since such light-dispersing element is relatively large-sized and its optical as well as structural alignment with optical fibers can hardly be ensured.